26-04-2012, 10:18 AM
Hi Rob,
Yes, the BF194/5s are used extensively in the RF and IF sections, and can indeed fail. So far, they seem slightly more reliable than the BC148s, but I guess there are factors at play - first, the sort of things the transistors are exposed to (collector currents, operating temperature, environmental stuff), and next, the impact the transistors are able to have. A slightly noisy transistor at the input stage is ruinous, whereas if that same noisy transistor is being used as a driver, or perhaps a VBE multiplier, then that noise wouldn't be noticed. Indeed, when trying to preserve originality, I've been known to swap transistors around rather than replace from stock.
The freezer tip is a good one - heat from a soldering iron can also isolate them. Incidentally, freezing that faulty resistor make no difference to the noise level...
As for transformers in audio amps, I accept they make things a lot simpler to understand
. But, they are quite bulky and expensive - well the good ones, at least, and they do add distortion - allied to that, because of phase shifts, with an amplifier with transformers, you are limited in the amount of corrective negative feedback you can add. Given that designers are tasked with getting good performance at low cost, you can see how these things evolved...
All the best,
Mark
Yes, the BF194/5s are used extensively in the RF and IF sections, and can indeed fail. So far, they seem slightly more reliable than the BC148s, but I guess there are factors at play - first, the sort of things the transistors are exposed to (collector currents, operating temperature, environmental stuff), and next, the impact the transistors are able to have. A slightly noisy transistor at the input stage is ruinous, whereas if that same noisy transistor is being used as a driver, or perhaps a VBE multiplier, then that noise wouldn't be noticed. Indeed, when trying to preserve originality, I've been known to swap transistors around rather than replace from stock.
The freezer tip is a good one - heat from a soldering iron can also isolate them. Incidentally, freezing that faulty resistor make no difference to the noise level...
As for transformers in audio amps, I accept they make things a lot simpler to understand
. But, they are quite bulky and expensive - well the good ones, at least, and they do add distortion - allied to that, because of phase shifts, with an amplifier with transformers, you are limited in the amount of corrective negative feedback you can add. Given that designers are tasked with getting good performance at low cost, you can see how these things evolved...All the best,
Mark
